Humidity is one of the most important environmental factors in cannabis cultivation. Both excessively high and low humidity can affect growth, lead to pests and mold, and ultimately reduce yield and bud quality.
- Ideal humidity per growth phase
Cannabis has a different optimal relative humidity (RH) at each stage of the cycle:
Phase Ideal RV Note
Seedling 65–70% Stimulates root development and growth.
Vegetative stage 50–70% Higher humidity promotes leaf growth.
Flowering (beginning) 40–60% Reduces risk of mold.
Flowering (end) 40–50% Prevents botrytis and mold.
- Problems due to high humidity
Causes:
Poor ventilation.
Too wet soil or overwatering.
High humidity outdoors or in greenhouses.
Implications:
Fungus such as botrytis (bud rot) and powdery mildew.
Reduced photosynthesis and slower growth.
Odor problems and reduced top quality.
Solutions:
Improve ventilation and air circulation.
Use a dehumidifier in high RH conditions.
Avoid overwatering the substrate.
Space plants apart to promote airflow between buds.
- Problems due to low humidity
Causes:
Dry heating or air conditioning.
Low RH outside the grow room.
Insufficient watering of seedlings.
Implications:
Leaves curl, wilt or become brittle.
Slow growth of young plants.
Stress to the plant, which can reduce yield.
Solutions:
Use a humidifier or water bowls.
Mist water in the air around seedlings.
Keep the temperature in balance with the humidity.
- Humidity and mold prevention
Mold is often the biggest problem when the RH is too high, especially during flowering. Preventative measures:
Ensure there is sufficient distance between plants.
Remove dead or withered plant material.
Ventilate the room regularly.
Keep RV around the recommended values per phase.
Tip: A hygrometer in each growing zone helps to accurately monitor and adjust humidity.
- Combination of humidity and temperature
Humidity works closely with temperature: warm air can hold more moisture than cold air.
High temperature + high RH = dangerous for mold.
Low temperature + low RH = stress for the plant.
It is essential to find a balance between both factors for optimal growth.